Peracetic acid, sometimes called peroxyacetic acid or PAA, is a well known chemical for its strong oxidizing potential. Peracetic acid has a molecular formula of C2H4O3 or CH3COOOH, a molecular mass of 76.05 g/mol, and a molecular structure as follows:

Peracetic acid is a liquid with an acrid odor and is normally sold in commercial formulations as aqueous solutions typically containing, e.g., 5, 15 or 35 wt % peracetic acid. Such aqueous formulations not only contain peracetic acid but also hydrogen peroxide (e.g. 7-25 wt %) and acetic acid (e.g., 6-39 wt %) in a dynamic chemical equilibrium.
Peracetic acid is commonly manufactured by reaction of acetic acid and concentrated hydrogen peroxide, often with an acid catalyst, e.g., sulfuric acid, in a semi-continuous process that optimizes reaction time versus high yields, yet typically proceeds for days:
The reaction rate is proportional to the concentration of the reactants present, so preparation of less concentrated solutions of peracetic acid, e.g., less than about 10 wt % peracetic acid and especially less than about 1 wt % peracetic acid, typically require extremely long reaction times of many days for the reaction to reach completion.
As a result of the reaction kinetics described above, dilute concentrations of peracetic acid are normally prepared by water dilution of more concentrated peracetic acid solutions. However, dilution of concentrated peracetic acid solutions with water usually results in the hydrolysis of some of the peracetic acid and its decomposition into acetic acid, which reduces the amount of available peracetic acid. In addition, equilibration of the peracetic acid in the diluted solution with the other components present, hydrogen peroxide, acetic acid and water in the aqueous solution, may require many hours, if not days, before an equilibrated solution is obtained.
Various dilution and/or reaction procedures for obtaining dilute peracetic acid solutions are described in the literature, with the objective of shortening the overall time required to obtain a dilute peracetic acid solution, as compared with direct reaction of dilute acetic acid (or acetic anhydride) with dilute hydrogen peroxide.
Greenspan et al., in Proc. 42nd Ann. Mtg. Chem. Spec. Man. Assn. December 1955, pp. 59-64, concerns peracetic acid aerosols useful in bacteriological applications and discloses that peracetic acid is considerably less stable than hydrogen peroxide. The reference teaches that dilute peracetic acid solutions present special stability problems and that dilute, e.g. 1%, peracetic acid solutions prepared by dilution of concentrated peracetic acid with water will hydrolyze, making them unsuitable for sale as commercial formulations. Greenspan et al. disclose that stable dilute peracetic acid solutions can be made by use of peracid stabilizers in conjunction with proper adjustment of the relative concentrations of the components of the dilute peracid solution but provide no examples. A typical peracetic acid formulation used in the aerosol work was said to contain 1.0% peracetic acid, 14.5% acetic acid, 5.0% hydrogen peroxide, 1.0% sulfuric acid and 78.5% water.
U.S. Pat. No. 4,297,298 of Crommelynck et al. discloses a two step process for the production of dilute solutions of peracids such as peracetic acid. The first step (in the case of peracetic acid) involves the production of concentrated peracetic acid from the reaction of acetic acid or acetic anhydride with concentrated hydrogen peroxide in the presence of a strong acid catalyst like sulfuric acid, which can take up to 48 hours. The second step involves diluting the concentrated peracetic acid reaction mixture with one or more of the reagents to a non-equilibrium concentration, to retard the catalytic effect of the strong acid catalyst. The diluted non-equilibrium reaction mixture still proceeds to form additional peracetic acid, albeit at a much slower rate.
U.S. Pat. No. 5,349,083 of Brougham discloses a two step industrial process for the production of dilute solutions of peracids such as peracetic acid. In the first step (in the case of peracetic acid) concentrated acetic acid is reacted with concentrated hydrogen peroxide, optionally in the presence of an acid catalyst like sulfuric acid, to produce a non-equilibrium concentration of peracetic acid. In the second step, the non-equilibrium reaction mixture is diluted with water or, less preferably, with water and quantities of acetic acid and/or hydrogen peroxide, to reproduce an equilibrated dilute peracetic acid solution. Although this process reduces the overall time to produce the dilute peracid solution, many hours are still required to produce the intermediate, non-equilibrium peracid solution that is subsequently diluted.
U.S. Pat. No. 5,368,867 of Da Silva et al. teaches the accelerated production of dilute equilibrated, storage stable solutions of peracetic acid by employing a two step procedure. In a first step, a concentrated peracetic acid solution is diluted with water and only partially hydrolyzed (in the presence of an acid catalyst); the hydrolysis reaction is not allowed to reach equilibrium. In a second step, the hydrolysis reaction is quenched by addition of hydrogen peroxide to yield a low concentration of peracetic acid, containing between 0.05-2.5 wt % peracetic acid. The advantage cited for this invention is the reduction in time necessary for obtaining a stable dilute solution, in equilibrium, of peracetic acid in low concentrations; the examples illustrate holding times of the order of 2-5 days, compared to much longer times (6 days or longer) required in previous prior art methods.
U.S. Pat. No. 5,977,403 of Byers discloses a two step process for the production of dilute solutions of peracids such as peracetic acid. In the first step (in the case of peracetic acid) acetic anhydride is reacted with concentrated hydrogen peroxide in the presence of an acid catalyst like sulfuric acid for less than 60 minutes, to produce a non-equilibrium concentration of peracetic acid. In the second step, the reaction mixture, which has not reached equilibrium, is diluted with water and hydrogen peroxide to produce peracetic acid at a concentration of about 0.5 to about 15.0 wt % that is at or near equilibrium.
There remains a need for a direct, fast and cost effective method for producing dilute equilibrated aqueous peracetic acid solutions from more concentrated peracetic acid solutions, avoiding the multi-step and time-consuming procedures of the prior art.